Disposable working electrode for an electrochemical cell

ABSTRACT

A flow-through electrochemical cell assembly with a disposable working electrode structure, including (a) a perimeter wall defining a sample flow channel including an inlet and an outlet, (b) a sample inlet line in fluid communication with the sample flow channel inlet, (c) a sample outlet line providing fluid communication between the sample flow channel outlet and a remote reference electrode, and (d) a disposable working electrode structure comprising an electrically conductive and electrochemically active working electrode region bound as a layer, directly or indirectly, to an electrically insulating substrate surface. The substrate surface is in fluid-sealing relationship with the sample flow channel, and the working electrode region is in fluid communication with said sample flow channel. The working electrode is vapor deposited, directly or indirectly, onto the organic polymer substrate through a mask, and a fluid seal is formed between said working electrode region and perimeter wall.

BACKGROUND OF THE INVENTION

[0001] Flow-through electrochemical cells are used as detectors for avariety of separation systems including chromatographic and ionchromatographic systems. Dionex Corporation sells such electrochemicalcells under the trademarks ED40 and ED50 cells. Such cells include anamperometric working electrode in the form of a cylindrical wireembedded into a plastic block with the tip of the wire exposed to asample flow-through channel, typically enclosed by a plastic gasket heldin place under compression. These working electrodes are somewhatcomplicated and expensive to manufacture. After a period of use, theelectrode must be replaced or reconditioned by laborious polishing orother methods which can lead to a lack of reproducibility of thedetector output.

[0002] Thin film disposable electrodes have been used as in vitro testelectrodes and as in vivo implantable monitoring electrodes in a varietyof applications. See, for example, Michel, et al. U.S. Pat. No.5,694,932; Dahl, et al. U.S. Pat. No. 5,554,178; Saban, et al. U.S. Pat.No. 6,110,354; Krause, et al. U.S. Pat. No. 4,710,403; Grill, Jr., etal. U.S. Pat. No. 5,324,322; Kurnik, et al. U.S. Pat. No. 5,989,409;Diebold, et al. U.S. Pat. No. 5,437,999; Kuennecke, et al. WO 99/36786;Bozon, et al., Electroanalysis 13:911-916 (2001); Soper, et al.,Analytical Chemistry 72:642A-651 A (2000); Lindner, et al., AnalyticalChemistry 72:336A-345A (2000); Bagel, et al., Analytical Chemistry69:4688-4694 (1997); Madaras, et al., Analytical Chemistry 68:3832-3839(1996); and Marsouk, et al., Analytical Chemistry 69:2646-2652 (1997).However, none of the disposable electrodes described in these referencesare suggested for use in a flow-through electrochemical cell. Such cellshave unique requirements such as the requirement of minimal contributionto peak broadening and reference potential being independent of samplecomposition.

[0003] The minimal contribution to peak broadening is predominantlydetermined by a low value of “chromatographic dead volume.”

[0004] The independence of reference potential from solution compositionis realized only in “true” reference electrodes e.g. calomel or Ag/AgClequipped by a special type of electrolytic connection known as “saltbridge.” A typical salt bridge is a cylindrical container filled with a3 M KCl solution. The conductive connection to the reference half cellon one side and to the sample on the other side is realized using ionpermeable diaphragms.

[0005] All existing microfabricated cells employ either “pseudo”reference electrodes (e.g. palladium) or reference half cells withoutsalt bridges. The latter types of reference electrodes rely on aconstant concentration of chloride ions in a measured sample. Achievingsuch constant concentration of chloride ions is not practical underchromatographic conditions.

[0006] There is a need to provide a disposable and readily removableamperometric working electrode for a flow-through electrochemical cellwhich is less expensive to construct and is replaceable, thus avoidingthe potential lack of reproducibility incurred in reconditioningpermanent working electrodes.

SUMMARY OF THE INVENTION

[0007] In one aspect of the present invention, a flow-throughelectrochemical cell assembly is provided with a disposable workingelectrode structure. The assembly includes (a) a perimeter wall defininga sample flow channel including an inlet and an outlet, (b) a sampleinlet line in fluid communication with the sample flow channel inlet,(c) a sample outlet line providing fluid communication between thesample flow channel outlet and a remote reference electrode, and (d) adisposable working electrode structure comprising an electricallyconductive and electrochemically active working electrode region boundas a layer, directly or indirectly, to an electrically insulatingsubstrate surface. The substrate surface is in fluid-sealingrelationship with the sample flow channel, and the working electroderegion is in fluid communication with said sample flow channel. Theworking electrode structure is readily removable from saidelectrochemical cell assembly.

[0008] In another aspect of the invention, a method is provided formaking a disposable electrode structure and sample flow channel for suchan assembly. The method comprises the steps of (a) vapor depositingelectrically conductive and electrochemically active material, directlyor indirectly, onto an organic polymer substrate through a mask to forma pattern of a working electrode region, and (b) forming a fluid sealbetween said working electrode region and a perimeter wall to define afluid sample flow channel with said working electrode region in directfluid contact with said fluid sample flow channel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is an exploded schematic view of an electrochemical cellassembly according to the invention including a disposable electrode.

[0010]FIG. 2 is a top view of a masking for vapor deposition of theelectrode onto a substrate.

[0011]FIGS. 3a-3 c are schematic representations of a method for maskinga disposable electrode of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Referring to FIG. 1, a flow-through electrochemical cell detectoris illustrated including one embodiment of a disposable workingelectrode according the present invention. Most of the components ofthis cell can be similar to a conventional electrochemical cell such asthe ED40 cell of Dionex Corporation, with the exception that thedisposable working electrode structure replaces a generally permanentelectrode structure which is periodically reconditioned as by polishing.

[0013] In general terms, the flow-through electrochemical cell includesa sample flow channel in contact with a working electrode. Sampleanalyte in a liquid eluent solution flows through the sample flowchannel and from there through a reference electrode chamber. Electrodesurface reactions are carried out on the working electrode, typicallyincluding an electrically conductive and electrochemically activematerial which is in direct contact with the sample solution flowingthrough the sample flow channel.

[0014] Specifically referring to FIG. 1, in one embodiment of anelectrochemical cell assembly 10, a conventional reference electrodeblock 12 defines a contained cylindrical reference electrode chamber 14through which the sample solution flows passing through the sample flowchannel. Another suitable conventional electrode is in the form of acounter or auxiliary electrode 16.

[0015] The basic function of the auxiliary electrode is to prevent theelectrical current from running through the reference electrode. This isachieved by means of so-called three-electrode potentiostats. See pages47-48, 239-241, William R. LaCourse, Pulsed Electrochemical Detection inHigh-Performance Liquid Chromatography, John Wiley, New York 1997, pages47-48 and 239-241.

[0016] If the passage of the current through the reference is notminimized, oxidation or reduction of the reference material can takeplace (e.g. AgCl reduced back to silver or Ag oxidized to silver oxide)or change of chloride concentration in the junction solution which mayresult in a poor constancy of the reference potential. Thethree-electrode potentiostats were introduced in the 1950s. Prior tothat only two-electrode cells were in general use for voltammetry (i.e.measurement of current while controlling the potential)

[0017] As illustrated in FIG. 1, the sample flow channel in contact withthe working electrode is defined by a gasket 18 which is retained insealing relationship between the lower wall of counter electrode 16 andthe upwardly facing wall of disposable working electrode structure 20 tobe described hereinafter. Gasket 18 defines an interior cut-out forminga perimeter wall around sample flow channel 18 a. The configuration ofthe sample flow channel 18 a is defined by the thickness of gasket 18,and the length and width of the cut-out, preferably in the form of anelongated flow-through slot. As illustrated, the working electrodestructure 20 includes a support substrate 20 a, preferably formed of anorganic polymer, and includes an electrically conductive andelectrochemically active working electrode region 20 b, preferable inthe form of a thin layer, in a circular shape as illustrated. As will bedescribed hereinafter, the working electrode region 20 b is preferablyformed by vapor deposition of an electrically conductive andelectrochemically active material, directly or indirectly, ontosubstrate 20 a. As used herein, “electrochemically active” meansmaterial suitable for facilitating the required electrochemicalreactions for detection in electrochemical cells.

[0018] In the embodiment of FIG. 1, working electrode structure 20 alsoincludes an electrically conductive contact region 20 c, suitably alsoin the form of a circular disk, and an electrically conductive lead 20 dinterconnecting working electrode region 20 b and contact region 20 c.In a preferred embodiment, working electrode region 20 b, contact region20 c and lead 20 d are formed by vapor deposition of the sameelectrically conductive material directly or indirectly onto substrate20 a through a mask. As will be described hereinafter, an adhesion layerpreferrably is first deposited onto an organic substrate to facilitatebinding of the electrode material. Preferably the adhesion material isof the same configuration as regions 20 b and 20 c and lead 20 d and isalso formed by vapor deposition through a mask of substantially the sameshape. As in a conventional electrochemical cell, the assembly includesa working electrode connection 22, suitably spring loaded and inelectrical communication at one end of a potentiostat, including avoltage or current source, and at the other end in electrical contactwith region 20 c to establish an electrical connection with workingelectrode region 20 b through lead 20 d.

[0019] As illustrated, the working electrode region 20 b is disposed inthe sample flow channel 18 a in direct contact with sample flowingtherethrough. In an illustrated embodiment, connection pin 22 andcontact region 20 c are disposed to the exterior of sample flow channel18 a out of fluid contact with liquid flowing through the flow channel.This has the advantage of simplicity. The working electrode, connectorand contact pad are located in a planar arrangement on the same side ofthe polymeric substrate. This makes it possible to manufacture theentire working electrode in what is essentially a two-step deposition(e.g. with Ti and Au).

[0020] In contrast, the manufacturing of permanent electrodes requiresmany more steps: machining of a kel-F block, machining of a steelsupport plate, covering of a gold wire by a suitable insulatingmaterials, machining of a Teflon ferrule for the liquid seal between thegold wire and the Kel F material, machining of the gold contact padcylinder, insertion of the gold wire and of the contact pad into theopening in the Kel F material. Curing of the conductive polymer betweenthe electrode wire and the contact pad cylinder. Sanding down the goldwire to the level of the KelF material. Machine lapping of the goldwire, hand-polishing of the gold wire. Of these multiple steps, the handpolishing is very person-dependent and notorious for its lack ofreproducibility. The components are suitably held in the assembly undercompression using a holder block 24 which maintains gasket 18 andelectrode structure 20 in fluid sealing relationship. As illustrated,the compression is accomplished by the use of conventional wing nuts 26or other clamping means. In one alternative form, not shown, gasket 18can be formed integral with or adhered to substrate 20 as by an adhesivebond therebetween forming an integral unit which can be readily removedfrom the cell and replaced by another integral unit. Alternatively,gasket 18 can be mounted to counter electrode 16 or other supportstructure. In each of these or other possible configurations, adisposable electrode structure can be removed from the assembly andreplaced alone or in combination with a gasket and support plate orholder block.

[0021] Gasket 18 typically is flexible with a thickness in the range ofabout 0.01 to 0.0005 inch consists of a fluoro polymer such as Teflon®or such polymeric materials as polyetherimide or nylon.

[0022] A similar type of gasket can be used as is used in the ED40electrochemical cell. Such a gasket suitably includes an elongate slotfor flow channel 18 a, suitably 0.5 to 10.0 mm, preferably 0.8 to 5 mmlong. The channel width is suitably 0.1 to 3 mm, preferably 0.5 to 1.5mm. The gaskets are suitably 0.005 to 0.5 mm, preferably 0.013 to 0.1 mmthick.

[0023] As illustrated, the gasket can be held in place by bolts passingthrough openings in the gasket material at both ends of the gasket.

[0024] For use with disposable electrodes it is advantageous to modifythe outer shape of the ED40 cell gasket as illustrated in FIG. 1. Anelongated partial protrusion or tab covering the lead between theelectrode and the contact pad improves the liquid seal. Also ofadvantage is to use thicker (>0.05 mm) and/or softer materials (PTFE)for gasketing of disposable electrodes.

[0025] In one embodiment, the gasket can also be made an integral partof the disposable electrode. The polymeric gasket can be permanentlyattached to the disposable electrode. This can be done either by oxygenplasma treatment of both surfaces followed by pressing the gasketagainst the electrode at room temperature. Alternatively, a permanentbonding of gaskets and electrodes can be achieved by using polyethylenecoated polyester material of suitable thickness as a gasket. Aftercutting the material to the proper gasketing shape, the gasket ispressed to the face of the disposable electrode at a suitable elevatedtemperature, usually about 140° C.

[0026] Typically, the sample containing separated analytes in an eluentsolution flows through conventional fittings, not shown, from achromatographic separator, such as a packed bed chromatography columnupstream of the electrochemical cell to flow channel 18 a. The samplesolution flows through inlet tubing connected to a sample flow channelinlet, not shown, in the path illustrated by arrows 28. As in the ED40cell, the inlet can be formed by a pin hole opening through counterelectrode 16 in the upstream end of flow channel 18 a. The solutionflows across flow channel 18 a and exits through a sample flow channeloutlet in the path illustrated schematically by arrows 30 and flowsthrough a pin hole size opening, not shown, in counter electrode 16 intochamber 14 and exits chamber 14 through a fitting, not shown, throughchamber outlet 32.

[0027] In another system, a conventional chemical or electrochemicalsuppressor is disposed between the electrochemical cell detector and thechromatography separator of an ion chromatography system.

[0028] The working electrode region is disposed within flow channel 18 ato contact the flowing sample in eluent solution therein. A preferredway to accomplish this and to provide electrical contact with connectorpin 22 is to space contact region 20 c from working electrode region 20b and to interconnect them by lead 20 d. This can be accomplished by theuse of a mask which includes these three elements vapor depositedthrough the mask. In this configuration, the three elements arepreferably in the form of thin film bound directly or indirectly tosubstrate 20 a.

[0029] Referring to FIG. 2, a top view of a mask 40 designed for vapordepositing multiple electrode region is illustrated. Mask includesalignment holes 41 to hold the screen in place and fixing screw 42together with a fixing bar 44. In one embodiment, the mask 40 isprepared by wet etching of aluminum or stainless steel sheets. Theelectrical pattern is defined by openings in the mask. One way to vapordeposit the electrode region is by placing a sheet of polymericsubstrate between mask 40 and a stainless steel plate, not shown. Themask includes working electrode opening 40 b which defines workingelectrode region 20 b, larger contact region 40 c which defines contactregion 20 c and slot opening 40 d which defines lead 20 d. Suitable maskmaterials include metal (e.g. stainless steel, molybdenum), glass,quartz and silicon.

[0030] The metallic pattern may be prepared by conventional microfabrication techniques used in semi-conduction manufacture as described,for example, in M. Madou, Fundamentals of Microfabrication, CRC Press,New York, 1997. These methods include but are not limited to physicalvapor deposition (PVD) and chemical vapor deposition (CVD).

[0031] Preferably, before depositing the electrode region, an adhesionlayer is deposited using the mask 40. This method is illustratedschematically in FIGS. 3a-c. Referring to FIG. 3a, a thin film of theadhesion layer 50, illustrated as the darkened region 50 in FIG. 3b, issputtered through opening 40 b, 40 c, 40 d and mask 40 as by sputteringusing a high vacuum with Ar plasma. Such a technique is illustrated inM. Madou, Chapter 2, p. 60, FIG. 2.8 of Fundamentals of Microfabrication(CRC, 1997). Thereafter, as illustrated in FIG. 3c the mask ismaintained in place. A suitable electrode material for direct contactwith the sample in flow channel 18 b is vapor deposited as a second thinfilm 52 onto a surface of the adhesion layer 50. The advantage of theadhesion layer is that it improves the cohesion between the electrodelayer and the underlying substrate for any substrate, preferably anorganic polymer material.

[0032] Suitably the adhesion layer is formed of a material such astitanium, tungsten, chromium and alloys of these materials. A titaniumor tungsten titanium alloy adhesion layer is particularly effective toimprove an adhesion of a metallic working electrode layer to thepolymeric substrate. A typical thickness for the adhesion layer 50 isabout 50 Å to 5,000 Å.

[0033] A suitable electrode material in region 20 a is a metal,preferably a noble metal such as gold, platinum, copper or silver, oralloys thereof, although gold is the most frequently used one. Inaddition, a non-metallic electrode may be used for region 20 b such as acarboneous material (e.g. glassy carbon, graphite or carbon paste) incombination with an adhesion layer such as titanium. Similar sputteringtechniques would be employed. A typical thickness for the electrodematerial of layer 52 is about 100 Å to 10,000 Å.

[0034] A suitable top view configuration of working electrode region 20b is circular with a diameter of about 0.1 to 3 mm, and suitably about0.5 to 2.0 mm, preferably about 1 mm. A suitable contact region 20 c islarger because of the need to accommodate different types of usefulcontacting arrangements and to ensure good contact with pin 22.

[0035] Substrate 28 is preferably of a polymeric material with athickness in the range of about 0.002 to 0.020 inches. It is preferablyflexible for forming a good seal with gasket 18. Suitably, the polymericmaterial can be a polyester (such as polyethylene, terephthalate orpolyethylene naphthalate), polycarbonate, polyolefin, polyimide orpolyetherimide. Preferably, the polymeric material is a polyester (PENor PET-type) or a polycarbonate.

[0036] Other alternative structures for the disposable workingelectrodes include different geometrical shapes of the working electrodearea such as triangle, square or rectangle. Several possiblearrangements relative to the flow path are possible for each of thenon-circular geometries of the working electrodes. Also possible arecomb-like patterns of two or more “finger” shaped electrodes connectedto the same lead as the circular electrodes but protruding into the flowpath either in a parallel or in radial fashion. Also feasible areintercalated electrodes or two comb-like electrode patterns protrudinginto the flow path from the opposing sides.

[0037] The electronics connecting the system can be the onesconventionally used in a Dionex ED40 or 50 electrochemical cell. A truereference electrode, e.g., Ag/AgCl wire immersed in a reference solutionenclosed by suitable diaphragm or a glass membrane may be employed.

[0038] In one embodiment of the invention, microfabricated electrodesare used in conjunction with a salt bridge-equipped true referenceelectrode. The combination pH/Ag/AgCl electrode represents animprovement even over a “true” reference electrode. An integral part ofthe detection mechanism is a cyclical creation of a catalytic gold oxidelayer on the working electrode's surface. The IPAD mode freshly createsand removes the amino acid-detection-enabling gold oxide layer with afrequency of 1 Hz or higher. The creation of gold oxide is pH dependentand in consequence different levels of oxidation current are generatedas a detection background at different pH. With a Ag/AgCl referenceelectrode alone, any change of eluent pH, such as during achromatographic mobile phase gradient, results in a strongly slopingchromatographic baseline. With the glass-membrane equippedtrue-reference electrode such as pH/Ag/AgCl the reference potentialchanges with pH in an identical fashion as the rate of gold oxideformation. The pH-connected change of the reference potential is thusproviding an automatic compensation of the change of the oxidationcurrent. The resulting baseline during a pH gradient is then completelyflat.

[0039] In one embodiment of the invention, microfabricated electrodesare used with a pH compensated reference potential (i.e. true referenceelectrode, salt bridge, glass membrane).

[0040] The electrochemical cell of the present invention can be used inany application in which ED40 or ED50 cell is used. Thus, it can be usedto detect separated amino acids, sugars, amino sugars, amines, aminothiols or the like. One of the advantages of the working electrode andreference electrode of the present invention is that they are capable ofoff-setting the change of pH and thus to eliminate excessive base lineshifts. This is because of the built-in pH-related compensation ofoxidation currents.

[0041] An important advantage of the disposable electrode is that it canbe readily replaced after a single day or multiple day use at lowexpense before loss of performance of the cell.

[0042] The disposable electrodes of the present invention are compatiblewith a commercial low dead volume electrochemical cell. This enables useof a true reference or pH based reference potential.

[0043] A variety of samples were analyzed with different protocols usingan electrochemical cell with a disposable electrode according to thepresent invention. The chromatograms from such experiments were verycomparable to ones performed using the ED40 cell.

[0044] In order to more clearly illustrate the present invention, thefollowing examples of its practice are presented.

EXAMPLE 1

[0045] This illustrates a method for forming a sputtered thin film oftitanium and gold on a polymeric substrate according to the invention.

[0046] 1. Assembly of Polymeric Substrate, Stainless Steel Base Plateand Stainless Steel Masks for Coating

[0047] Polymeric film substrates obtained from Du Pont or GE werecleaned of all particles on their surface by blowing off with air,rinsed successively with water, alcohol and then dried in air. Afterpunching the holes required for mounting the masks on top of the film,the polymeric substrates were put on top of a stainless steel baseplate. We then placed first a thinner stainless steel mask and then athicker stainless steel mask on the exposed side of the polymeric film.The patterns of the thinner mask is shown in FIG. 2. The thinner maskdefines the shape of the electrode, connection lead and contact pad. Thethicker mask, not shown, is used for keeping the thinner mask flat,completely co-planar and in close contact with the polymeric film. Atthe same time, the thicker mask has open cutout areas, thus providingthe structural integrity without interference with the plasma during thesputtering of titanium and gold. The polymeric films are sandwichedtightly between the two masks and the supporting base plate. The wholeassembly is being held together by bars and screws. The bars arepositioned on top of the two masks.

[0048] 2. Physical Vapor Deposition of Titanium and Gold

[0049] The polymeric substrates assembled with masks are placed in thesputtering chamber. A suitable vacuum is applied for 12 hours(overnight) to reach the vacuum required for sputtering (at least 40mTorr). The water adsorbed inside the polymer is slowly removed from thechamber during that time. To initiate the deposition, the substrateremains enclosed in a low-pressure gas atmosphere (ca. 10 mTorr ofargon). For RF plasma deposition the substrate is connected as anode andthe metal source for deposition (target) is connected as cathode. Asuitable RF frequency is within the range of 12-14 mHz. The suitablerange of RF power is in the range of 1 to 2 kW. The deposition rate isdifferent for different metals. For the same frequency and power of theRF field, titanium deposition is ca. 4.7 times slower than thedeposition of gold (see for example Table 3.8, page 100, M. Madou,Fundamentals of Micromachining). The RF field generated between thesubstrate and target is the sole heating source during the metaldeposition. The temperature of the polymeric substrate never exceeds therange of 50-70° C.

[0050] A titanium layer is sputtered first to promote adhesion of goldfilms to polymeric substrates. A typical thickness of the first metalliclayer is 50 to 1000 Å. The layer of titanium is the onlyadhesion-promoting agent utilized in our process. There are no otheradhesives being utilized to promote adhesion of gold layer to thepolymeric substrate. The second layer (Au) is usually 100 to 5000 Åthick. The sputtering time varies from system to system because thecoating rate depends on the power of the radio frequency (plasmasource), the distance between the polymeric film and target (source ofmetal being deposited) and others.

EXAMPLE 2

[0051] Assembling a Suitable Cell

[0052] (1) Remove the ED40 cell body made of titanium from the stainlesssteel box serving as a Faraday Cage/electrode mounting container andunscrew the steel cylinder holder for the reference electrode.

[0053] (2) Verify that a black O ring (Viton) is in place in the lowerpart of the reference electrode chamber.

[0054] Insert a pH/Ag/AgCl reference electrode (glass cylinder) into thereference electrode chamber of the cell body.

[0055] (3) Install the steel cylinder holding the reference electrode inpre-defined position inside the reference electrode chamber.

[0056] (4) Connect the lead wires of the reference electrode to the “pH”and “Ag” pins of the pre-amplifier board.

[0057] (5) The white cable of the working electrode connection remainsconnected to the two “WE” pins.

[0058] (6) Unscrew the two winged screws and remove the permanentworking electrode from the cell body.

[0059] (7) Remove the standard cell gasket and replace it by a cellgasket for use with disposable electrodes.

[0060] (8) Match the two holes of the disposable electrode unit (outsidedimensions 2.5×3 cm) to the two posts protruding from the cell body. Thetwo openings of the disposable electrode match the distance between thetwo posts (2 cm). Slide the disposable electrode all the way to thebottom of the two alignment posts. This positions the working electrodecorrectly inside the flow path defined by the gasket cutout. Make surethat the metallized side of the disposable electrode unit faces theelectrode cell body and the gasket. The correct position of the workingelectrode can be verified through the transparent polyester substrate ofthe disposable electrode. The correct orientation of the disposableworking electrode is indicated by the titanium color (not gold) beingvisible through the polyester when the unit is in the position close tothe cell body.

[0061] (9) Slide the permanent electrode (or alternatively a lessexpensive holder block) onto the two posts pressing the disposableelectrode against the cell body. Check visually the presence of the cellgasket and the correct contact between contact pin and contact pad.

[0062] (10) Mount the two winged nuts.

[0063] (11) Make liquid connections to and from the electrode cell.

[0064] (12) Slide the steel mounting box/Faraday Cage over the assembledcell.

[0065] (13) Connect the assembled cell to the electronic unit of theED40 detector.

[0066] (14) Start the pump and wait until you see the first drops comingout of the outlet capillary.

[0067] (15) Check the pH readout on the screen of the ED40 electronicunit.

[0068] (16) Apply a suitable detection potential or detection waveform.

1-15. Cancelled.
 16. A method for making a disposable working electrodestructure and a sample flow channel for use in an electrochemical cellassembly, said method comprising: (a) vapor depositing electricallyconductive and electrochemically active material, directly orindirectly, onto an organic polymer substrate through a mask to form apattern of a working electrode region, and (b) forming a fluid sealbetween said working electrode region and a perimeter wall to definesaid fluid sample flow channel with said working electrode region indirect fluid contact with said fluid sample flow channel.
 17. The methodof claim 16 in which said vapor deposition is through said mask whichmask forms a pattern of an electrically conductive lead interconnectingsaid working electrode and an electrically conductive contact regionforming said disposable working electrode structure.
 18. The method ofclaim 16 further comprising, before step (a), vapor depositing anadhesion layer onto said organic polymer substrate through a mask,wherein step (a) is performed by vapor depositing said electricallyconductive material and electrochemically active onto said adhesionlayer.
 19. The method of claim 18 in which said adhesion layer is formedof a material selected from the group consisting of titanium, tungsten,chromium, and alloys thereof.
 20. The method of claim 16 in which saidorganic polymer is selected from the group consisting of polyester,polycarbonate, polyolefin, polyimide and polyetherimide.
 21. The methodof claim 16 in which said vapor depositing step includes forming saidpattern of said working electrode region to be approximately 100 Å to10,000 Å thick.
 22. The method of claim 16 in which said workingelectrode region is bound to said substrate by an intermediate adhesionlayer.
 23. The method of claim 22 in which said intermediate adhesionlayer is approximately 50 Å to 5000 Å thick.
 24. The method of claim 16in which at least a surface portion of said substrate is exposed to saidsample flow channel.
 25. A method for making a flow-through electricalcell assembly comprising: vapor depositing electrically conductive andelectrochemically active material, directly or indirectly, onto anorganic polymer substrate through a mask to form a pattern of a workingelectrode region; providing a reference electrode including a wallhaving an inlet and an outlet spaced therefrom; mounting a sealingmember to said wall to define a sample flow channel fluidly couplingsaid inlet and said outlet; and positioning said substrate, and saidworking electrode region thereon, on said sealing member such that saidworking electrode region is in fluid communication with said sample flowchannel.
 26. The method of claim 25 in which said vapor depositionthrough said mask forms a pattern of an electrically conductive leadinterconnecting said working electrode and an electrically conductivecontact region forming said disposable working electrode structure. 27.The method of claim 25 further comprising vapor depositing an adhesionlayer onto said organic polymer substrate through a mask, wherein saidvapor depositing step is performed by vapor depositing said electricallyconductive material and electrochemically active onto said adhesionlayer.
 28. The method of claim 27 in which said adhesion layer is formedof a material selected from the group consisting of titanium, tungsten,chromium, and alloys thereof.
 29. The method of claim 25 in which saidorganic polymer is selected from the group consisting of polyester,polycarbonate, polyolefin, polyimide and polyetherimide.
 30. The methodof claim 25 in which said vapor depositing step includes forming saidpattern of said working electrode region to be approximately 100 Å to10,000 Å thick.
 31. The method of claim 25 in which said workingelectrode region is bound to said substrate by an intermediate adhesionlayer.
 32. The method of claim 31 in which said intermediate adhesionlayer is approximately 50 Å to 5000 Å thick.
 33. The method of claim 25in which at least a surface portion of said substrate is exposed to saidsample flow channel.